K. Makishima, T. Ohashi, N. Kawai, M. Matsuoka, K. Koyama, H. Kunieda, Y. Tawara, N. Ushimaru, R.H.D. Corbet, H. Inoue, T. Kii, F. Makino, K. Mitsuda, F. Nagase, Y. Ogawara, Y. Tanaka, S. Kitamoto, S. Miyamoto, H. Tsunemi, and K. Yamashita
Tenma observations were made of the transient 4.4-s X-ray pulsar X0331 + 53 (V0332 + 53) at several phases of its 34-d binary orbit during the period November 1983-January 1984. The measured X-ray flux and pulse periods agree with those obtained with Exosat. Rapid random fluctuations in the X-ray emission from this source were studied. It appears this type of variability is not rare among accreting pulsars, but is particularly noticeable in X0331 + 53 due to the small pulse fraction. The X-ray spectra show a turn-over above about 15 keV, which is better described by a recently introduced cyclotron absorption formula than the commonly used exponential cutoff model. In terms of this model, a magnetic field strength of about 3 x 10 to the 12th G is suggested for X0331 + 53. The absence of iron K-emission lines in the spectra indicates a rather small circumstellar matter density. This, together with the observed high luminosity (about 3 x 10 to the 36th erg/s) and large intensity modulation along the orbit, suggests that the pulsar is powered by accretion from the equatorial envelope of the supposedly Be-type primary (BQ Cam).